1. Field of the Invention
The present invention relates to current regulating apparatus for providing regulated direct current to the magnetizing coils of a non-destructive testing apparatus used to detect welding defects in pipe.
2. Description of the Prior Art
A non-destructive testing apparatus for detecting welding defects in pipe as the pipe is moved at variable speeds down a mill run is shown in U.S. Pat. No. 3,753,085, having a substantially increased signal-to-noise ratio over prior non-destructive testing apparatus and permitting more positive detection of defects in both longitudinal and circular welds.
In these apparatus, variations in the speed of the pipe as it moved down the mill run induced voltage variations in the detector signal output. In order to provide a signal output with a substantially high voltage to accurately indicate a weld defect, the current level supplied to the magnetizing coils had to be sufficiently high to induce signal output voltages larger than the voltages caused by the variations in pipe speed. However, if the current level became too high, the signal-to-noise ratio of the detector output deteriorated below the high level required for accurate testing. Thus, for highly accurate testing results, current levels supplied to the magnetizing coils were required to be maintained within a relatively narrow range. In such testing apparatus, the problem was further compounded in that the energizing current for the magnetizing coils varied as the temperature of the coils increased during testing operations.
In the past, in order to prevent current variations and maintain current levels within the narrow range for accurate results, the current to the magnetizing coils had to be frequently adjusted manually in order to maintain the current levels required for proper operation of the testing apparatus.
So far as is known, prior art direct current sources for pipe testing apparatus were not capable of supplying a closely regulated current within the limited range required to provide a signal output with high signal-to-noise ratio and yet of a sufficiently large magnitude to compensate for pipe speed variations, particularly in a variable temperature environment.